That’s not a big surprise. The talks haven’t been going well, especially the negotiations with the pilots. The pilots, at least, are all represented by one union. Some of the other work groups are still deciding who will represent them.

United can push ahead and begin operating as one airline without officially combining the work groups, but that’s not a recipe for labor peace. Just ask US Airways.

Labor issues are one of the biggest stumbling blocks in airline mergers, and Smisek always seemed to underestimate the difficulty of combining unions.

“I originally set an admittedly aggressive goal of reaching joint agreements with all of our representative work groups by year-end,” he told analysts in a conference call today. “I don’t think we’ll reach that goal given the complexities of merging mature, long-standing contracts as well as factors outside of our control.”

As I’ve noted before, airline negotiations are largely conducted in public, and Smisek’s comments are designed to put pressure on the unions to get a deal done, but it’s also clear that things are not going smoothly.

From the beginning, this merger’s success hinged on how well the Continental culture could prevail in areas such as customer service and labor. It appears in the case of labor, things are slipping back towards United’s long history of labor strife.